Got passed by some clown a few weeks ago on a curvy section of the MUP that I would consider to be a "caution" area - if it were a race, there'd be a yellow caution flag out. So this dude passes me tightly on the left on a right turn where I am in the middle of the lane (taking the conservative line and not realizing anybody was behind me). He had just enough room to get by, but it was a dumb place to pass. I accelerated up next to him as the path broadened, then bumped him enough for him to swerve a bit before letting him know I didn't appreciate the timing or manner of his passing. I felt much better and accelerated away. Useful things you pick up after 40 years of riding track.

I get really annoyed by riders who insist on taking the "racing line" thru a blind corner. This means they're coming right at you in your lane when you make a right turn (or a left in the UK).

I've also found that females tend to be more guilty of spreading across the MUP and blocking it than males. And solo runners/joggers are usually wearing headphones, so they seldom hear the bell.

But you're correct that MUPs are not the place to be out training. Even protected bike lanes have to be taken at "caution" speeds!

Luis